The present invention relates to spark plugs for an internal combustion engine, and relates more particularly to connecting these plugs to the ignition circuit of the engine.
A spark plug generally has a center electrode and a side electrode, insulated from one another by a body of insulating material, such as ceramic, and the two electrodes having two mutually facing ends which define a spark gap.
At its opposite end, which is intended to be connected to the ignition circuit, the plug has a connection terminal connected to the center electrode.
This terminal is generally formed by a standardized piece, manufactured up to the present by two known techniques.
According to a first technique, the connection terminal is formed by a rod, the free end of which is threaded in accordance with ISO Standard 1919-1982. Mounted on this threaded end of the rod is a connection piece formed by a nut or threaded bush, in accordance with ISO Standard 1919-1982.
According to a second technique, the end rod of the electrode and the connection piece are made in one piece, by cold stamping or bar turning.
The connection terminal manufactured according to the first technique has the disadvantage of frequently coming unscrewed under the effect of the vibratory stresses of the combustion engine.
The connection terminal manufactured according to the second technique, the one-piece connection terminal, has the disadvantage, principally in the technique for mounting plugs referred to as "shrink-fitting" or "with conductive adhesive", of being covered by a layer of oxidized nickel, for example, which makes the fitting and removal of the electric supply leads of the plug particularly difficult.